1. Field of Invention
This invention is an ultrafast optical time division multiplexed optical communication system comprising an ultrafast electronic-to-optical multiplexer and an ultrafast optical-to-electronic demultiplexer.
2. Prior Art
A common problem with ultrafast optical communication systems is how to multiplex electronic rate signals up to optical rates and demultiplex the optical rate signals down to electronic rates.
One approach to implementing an electronic-to-optical multiplexer, as shown in FIG. 1 (prior art) converts electronic signals into optical signals and then uses optical switches to select the optical signal. One problem with this approach is that it takes n optical switches to multiplex n electronic signals.
One approach to implementing an optical-to-electronic demultiplexer as shown in FIG. 1 (prior art) uses optical switches to select a time slice of the time division multiplexed optical signal. The selected time slice is then detected and converted into an electronic signal. One problem with this approach is that it takes n optical switches to demultiplex n electronic signals.
One optical switch that has been used for multiplexing and demultiplexing is the Sagnac switch, also referred to as a nonlinear optical loop mirror (NOLM). This optical switch is discussed in the article, “All-optical arbitrary demultiplexing at 2.5 Gb/s with tolerance to timing jitter,” by N. Whitaker, et al., Optical Letters, vol. 16, pp. 1838-1840, 1991. Over the years, the multiplexing and demultiplexing rate has been pushed upwards. This advance is illustrated by the article, “1.28 Tbit/s-70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation with a phase modulator,” M. Nakazawa, et al., Electronics Letters, vol. 36, issue 24, p. 2027-2029, Nov. 23, 2000.
The Sagnac switch is discussed in more detail in “Low distortion all-optical threshold device,” H. Avramopoulos, et al., U.S. Pat. No. 5,146,517, Sep. 8, 1992; “Sagnac optical logic gate,” M. C. Gabriel, et al., U.S. Pat. No. 5,144,375, Sep. 1, 1992; in the background discussion of “Optical State Machines,” U.S. patent application 11380768; and in “Sagnac fiber logic gates and their possible applications: a system perspective,” by A. Huang, et al., Applied Optics, vol. 33, no. 26, pp. 6254-6267, Sep. 10, 1994.
Briefly, there are two versions of the Sagnac logic gate. One is based on polarization while the other is based on wavelength. The polarization based Sagnac logic gate is shown in FIG. 2 (prior art). The polarization based Sagnac switch comprises a 50/50 polarization maintaining splitter 11 with a first and second end each with a first and second bi-directional port each with a fast and slow axis of polarization; an “input” polarization selective coupler 13 with a first and second end each with a first and second bi-directional port each with a fast and slow axis of polarization; a loop of polarization maintaining fiber 14 with a first and second end each with a fast and slow axis of polarization; a “dump” polarization selective coupler 12 with a first and second end each with a first and second bi-directional port each with a fast and slow axis of polarization; a clockwise polarization maintaining optical circulator 10 with three bi-directional ports each with a fast and slow axis of polarization; a delay 15 with an first and second end each with a fast and a slow axis of polarization; and a delay 16 with a first and second end each with a fast and a slow axis of polarization.
The slow axis of polarization output of the second (clockwise) port of optical circulator 10 is connected to the slow axis of polarization of the first port of the first end of 50/50 polarization maintaining splitter 11. The slow axis of polarization of the first output of the second end of splitter 11 is connected to the slow axis of polarization first port of the first end of the “dump” polarization selective coupler 12. The slow axis of polarization of the first port of the second end of polarization selective coupler 12 is connected to the slow axis of polarization of the first end of the loop of polarization maintaining fiber 14. The slow axis of polarization of the second end of the loop of fiber 14 is connected to the slow axis of polarization of the first port of the second end of the “input” polarization selective coupler 13. The slow axis of polarization of the first port of the first end of the input polarization selective coupler 13 is connected to the second port of the second end of 50/50 polarization maintaining splitter 11.
Input A is connected to the slow axis of polarization of the first port of optical circulator 10. Output X is connected to the slow axis of polarization of the second port of the first end of 50/50 polarization maintaining splitter 11. Input B is connected to the fast axis of polarization of the second port of the first end of “input” polarization selective coupler 13. Output C is connected to the fast axis of polarization of the first port of the first end of the “dump” polarization selective coupler 12. Output Y is connected to the slow axis of polarization of the output of the third (clockwise) port of optical circulator 10.
The length of the optical fiber 14 is selected such that input B signal on the fast axis of polarization has enough time to completely pass through input A signal on the slow axis of polarization.
In the un-switched mode of operation, a first signal with a slow axis of polarization is fed to input A. Input A is connected via circulator 10 to splitter 11. Splitter 11 splits the signal into a clockwise and counter clockwise portion. The clockwise portion passes via “input” coupler 13, fiber loop 14, and “dump” coupler 12 to splitter 11. Splitter 11 splits the clockwise signal into two portions. One portion emerges at the first port of the first end of splitter 11; while the other portion emerges at the second port of the first end of splitter 11. The counter clockwise portion passes via “dump” coupler 12, fiber loop 14, and “input” coupler 13 to splitter 11. Splitter 11 splits the counter clockwise signal into two portions. One portion emerges at the first port of the first end of splitter 11, while the other portion emerges at the second port of the first end of splitter 11. The clockwise and counter clockwise portions of the input signal constructively interfere and reconstruct the input A signal at the first port of the first end of splitter 11. As a result, a reconstructed version of the input A signal emerges from output Y, sometimes called the normally connected output, via circulator 10. The clockwise and counter clockwise portions of the input A signal destructively interfere at the second port of the first end of splitter 11. As a result, no signal emerges from output X, sometimes called the normally open output. The un-switched mode of operation is commonly called the “mirror” mode, since the input signal fed to first port of the first end of splitter 11 emerges at same port and the Sagnac switch appears to act like a mirror.
In the switched mode of operation, a first input signal with a slow axis of polarization is fed to input A. Input A is connected via optical circulator 10 to splitter 11. Splitter 11 splits the input A signal into a clockwise and counter clockwise portion. The clockwise portion passes via the slow axis of propagation of “input” coupler 13, polarization maintaining fiber loop 14, and “dump” coupler 12 to splitter 11. Splitter 11 splits the clockwise signal into two portions. One portion emerges at the first port of the first end of splitter 11, while the other portion emerges at the second port of the first end of splitter 11. The counter clockwise portion passes via the slow axis of propagation of “dump” coupler 12, fiber loop 14, and “input” coupler 13, to splitter 11. Splitter 11 splits the counter clockwise signal into two portions. One portion emerges at the first port of the first end of splitter 11, while the other portion emerges at the second port of the first end of splitter 11. Meanwhile, a second input signal with a fast axis of polarization is fed to input B and passes via the fast axis of polarization of “input” coupler 13, fiber loop 14, and “dump” coupler 12 to output C. The input B signal co-propagates with the clockwise portion of the input A signal and counter-propagates with the counter clockwise portion of the input B signal. The ultrafast, nonlinear, all-optical Kerr effect shifts the phase of the clockwise portion of the input A signal relative to the counter clockwise portion of the input A signal. The amount of this phase shift is determined by the intensity of the input B signal and the amount of time the co-propagating clockwise input A signal and input B signal temporally overlap. The overlap time depends on the length of the loop and the difference in the index of refraction of the slow and fast axis of polarization of the couplers and fiber loop. The intensity of the input B signal and the physical parameters of the Sagnac switch are selected such that the input B signal induces a π or a 180 degree phase shift between the clockwise and counter clockwise portions of the input A signal. The clockwise and counter clockwise portions of input A signal then destructively interfere at the first port of the first end of splitter 11 that is connected via circulator 10 to output Y. As a result, no signal will emerge at output Y. Meanwhile, the clockwise and counter clockwise portions of input A signal constructively interfere and reconstruct the input A signal at the second port of the first end of splitter 11 which is connected to output X. As a result, a reconstructed version of the input A signal emerges at output X. The switched mode of operation is commonly called the “loop” mode of operation, since the input signal fed to the first port of the first end of splitter 11 emerges at the second port of the first end of splitter 11 and the Sagnac switch acts like a loop.
As mentioned previously, the Sagnac switch can also be based on wavelength. This implementation is discussed in the cited references.
The Sagnac switch has several useful properties relevant this invention: the input A and input B are capable of handling optical rate signals; the input A and input B signals do not have to be the same wavelength; the input A and input B signals do not have to arrive simultaneously; the input B signal can be distorted; and the input B signal can control a wavelength division multiplexed input A signal.